Method for measuring the thickness of flat workpieces

ABSTRACT

A method for measuring the thickness of flat workpieces processed in a double-side processing machine comprises processing the workpieces in a working gap formed between an upper working disk and a lower working disk configured to remove material from the workpieces. Optically measuring the thickness of the workpieces during processing by at least one optical thickness measurement apparatus disposed on at least one of the upper working disk or the lower working disk. The at least one optical thickness measurement apparatus configured to measure the thickness of the workpieces disposed in the working gap through at least one through-hole in a corresponding upper working disk or the lower working disk. Supplying the measurement results of the at least one thickness measurement apparatus to a control apparatus of the double-side processing machine. Terminating the processing of the workpieces once a previously specified target thickness of the workpieces is reached.

CROSS REFERENCE TO RELATED INVENTION

This application is based upon and claims priority to, under relevantsections of 35 U.S.C. §119, German Patent Application No. 10 2016 116012.1, filed Aug. 29, 2016, the entire contents of which are herebyincorporated by reference.

BACKGROUND

The invention relates to a method for measuring the thickness of flatworkpieces processed in a double-side processing machine. Flatworkpieces such as wafers, in particular silicon wafers, are processedin double-side processing machines, for example double-side polishingmachines to remove material from both sides of the flat workpiece.During this removal polishing of the workpieces, it is of vitalimportance to inspect the workpiece thickness in order to attain thequality requirements. The target window for achieving the best possibleworkpiece geometry is approximately 100 nm. If the target measure ispassed, for example because the processing operation takes too long, theedge geometry is generally not sufficient for the requirements of waferconsumers. In particular, high demands are placed on the flatness of theworkpieces. Typically, the requirement for the wafer geometry parameterSFQRmax, i.e. the maximum value of the local flatness (“site flatness”)on a silicon wafer, is 15 nm or even as low as 10 nm. During otherprocessing operations such as, for example, haze-free polishing or evengrinding, a precise knowledge of the respective workpiece thickness isalso required.

The process of measuring the distance of the working disks and,therefore indirectly the workpiece thickness, by means of eddy currentsensors is known. However, the accuracy requirements explained abovecannot always be achieved. In addition, this type of measurement isdependent on the thickness and the wear of a working coating on theworking disk such as a polishing cloth. It is therefore standardprocedure to predict the workpiece thickness based on a plurality ofexternal parameters such as the properties of the polishing agent(temperature, pH value, age, dilution, concentration of solids, solidparticle sizes) and the polishing cloth (nature of the conditioning,type of diamond dressing, flatness, shape, wear, glazing) as well asprocess and handling times. By inspecting these external parameters, anattempt is made to comply with process conditions, which are as constantas possible (a repeatable removal rate). The thickness is then inspectedvia the process time and a known preliminary measurement. The accuracyrequirements mentioned above cannot, however, be regularly achieved withthis method either. In particular, the external parameters change duringprocessing, for example, due to wear. This results in deviations fromthe predicted workpiece thickness. Therefore, an external thicknessmeasurement of the workpieces is generally performed followingprocessing in the double-side processing machine. This is complex, bothfrom a metrological and process technology point of view. In particular,the wafers must be cleaned in a cleaning plant prior to measurement.Depending on the production capacity of the double-side processingmachine, a substantial measurement capacity must be provided. Inaddition, a non-permissible thickness deviation can only be determinedin this way following processing. The process parameters of theprocessing machine are corrected at correspondingly staggered intervals,resulting in scrap.

A process of measuring the thickness of flat workpieces, such as wafers,by means of interferometry is generally known from WO 2010/037452 A1. Inthis way, the workpiece thickness can be established extremelyprecisely. For example, optical measuring methods for determining thethickness of flat workpieces are also known from US 2006/0037699 A1, EP1970163 B1, DE 112009001875 T5 or U.S. Pat. No. 6,437,868 B1. Thethickness measurement apparatus is fixed on the processing machine andmeasures the thickness of the workpieces processed in the machine, forexample through a measurement opening in a, for example, rotating partof the machine. It is of course only possible to take measurements ifthe optical axis of the measuring apparatus is aligned precisely withthe measurement opening. This is complex in design and processengineering terms, and only relatively few measuring points areavailable. This, in turn, adversely affects the reliability and accuracyof the measurement.

Starting from the explained prior art, the object of the invention istherefore to provide a method of the type indicated above, with which itis possible to determine the thickness of workpieces processed in adouble-side processing machine reliably and extremely precisely, using asimple process engineering method.

BRIEF SUMMARY OF THE INVENTION

The invention achieves the object through a method for measuring thethickness of flat workpieces processed in a double-side processingmachine. The method comprises processing the workpieces in a working gapformed between an upper working disk and a lower working disk of thedouble-side processing machine while the working disks rotate relativeto one another in a material-removing manner. Optically measuring thethickness of the workpiece during the processing of the workpiece bymeans of at least one optical thickness measurement apparatus arrangedon the upper working disk and/or the lower working disk, wherein the atleast one thickness measurement apparatus measures the thickness of theworkpieces located in the working gap through at least one through-holein the upper working disk and/or the lower working disk. Supplying themeasurement results of the at least one optical thickness measurementapparatus to a control apparatus of the double-side processing machine.Terminating the processing operation of the workpieces once a previouslyspecified target thickness of the workpieces is reached.

The double-side processing machine can, for example, be a double-sidepolishing machine. However, it can also be another double-sideprocessing machine such as a double-side grinding machine. Multipleworkpieces can be processed simultaneously in the working gap of thedouble-side processing machine, removing material on both sides of theworkpieces. To this end, the working disks are regularly provided with aworking coating, for example a polishing coating. Rotor disks, whicheach have openings for receiving one or more workpieces such assemiconductor wafers, in which the workpieces are held in a floatingmanner, are known. The rotor disks have teeth on their outercircumference. The teeth mesh with corresponding teeth on the outer sideand on the inner side of the working gap. As a result, the rotor diskrotates in the working gap and the workpieces are guided along cycloidtracks through the working gap. A particularly uniform processing can beachieved in this way. During the processing one of the working disks orboth working disks can then be rotatingly driven in opposite directions.Such double-side processing machines are themselves known.

According to the invention, the workpiece thickness is opticallymeasured by means of at least one optical thickness measurementapparatus fixed to the upper working disk and/or the lower working diskduring the processing of the workpieces in the working gap. To this end,the upper and/or lower working disk equipped with the thicknessmeasurement apparatus has a through-hole, through which the thicknessmeasurement apparatus, which also rotates with the respective workingdisk, detects the workpiece thickness. The through-hole thereforeextends from the optical thickness measurement apparatus right into theworking gap, in which the workpieces are located. The working disk canalso have a two-piece construction with a first part, which delimits theworking gap and a second part, which holds the first part in the mannerof a carrier disk. The thickness measurement apparatus can thenaccordingly also be attached to the second part, which is configured asa carrier.

As explained, the thickness measurement apparatus according to theinvention operates optically and accordingly achieves a high precision.To this end, it has an optical radiation source, for example a laser.The radiation from an optical radiation source can be coupled into alight guide, such as a fiber optic, which directs the optical radiation,if necessary, via a focusing optic comprising, for example, lensesthrough the through-hole onto the workpieces to be measured. Thethrough-hole or through-bore through which the thickness measurementapparatus measures can, be configured decentrally in the upper and/orlower working disk. It is understood that if a working coating isprovided on the working disk, said working coating must accordingly alsohave the through-hole so that the workpieces located in the working gapcan be measured.

The measurement results of the at least one thickness measurementapparatus detected during the processing are also supplied to a controlapparatus during the processing of the workpieces. Said controlapparatus can compare the measured values, for example, with a targetthickness previously specified for the workpieces. As soon as the targetthickness is reached the control apparatus can terminate the currentprocessing operation. This can be the end of the processing operation ofthe workpieces in the processing machine as a whole. However, it is alsopossible that following the end of the current processing operation, adifferent subsequent processing operation commences which is, in turn,controlled by the control apparatus. The measurement results can ofcourse also be processed prior to forwarding to the control apparatus.The measurement results can, for example, be initially supplied to aninterposed computer or similar, which further processes the thicknessvalues of the thickness measurement apparatus and ascertains thosethickness values which have been filtered, smoothed or processed in adifferent way, which are finally sent to the control apparatus. Thecomputer or similar can, in this respect, also be part of the controlapparatus.

The advantage of the fixed connection according to the invention of thethickness measurement apparatus having the working disk is that, even inthe case of a single through-hole in the working disk, a light pathwhich is not interrupted by the rotating working disk is directed intothe process area having the workpieces, i.e. the working gap, at anytime. Therefore, as opposed to measuring apparatuses which are arrangedin a non-rotating manner on a frame or housing of the machine, such asthose proposed in the prior art, it is also possible to take thicknessmeasurements continuously. In any case, the scope and the number of themeasured values obtained are not restricted in design terms, as is thecase in the prior art.

In addition, the advantage of the invention is that the through-hole canbe sealed from the side of the thickness measurement apparatus so thatdirt and moisture cannot get into the light path or the focusing opticof the thickness measurement apparatus. At the same time, the directoptical thickness measurement according to the invention makes itpossible to detect the thickness precisely in situ during processing,wherein an elaborate preliminary measurement and an external subsequentmeasurement are dispensed with. Rather, the control apparatus canautomatically terminate the processing operation after reaching thepredefined target thickness or a predefined minimum removal. Due to thereliable and precise in-situ determination of the workpiece thicknessaccording to the invention, measuring capacities on elaborate externalmeasuring instruments are freed up. The production logistics aresimplified, as it is no longer essential to know the process history.Precisely continuous operation in order to stabilize the rate of removalis no longer absolutely necessary, reducing the volume of scrap.

As already mentioned, the method according to the invention can be usedin a particularly advantageous way during the processing of wafers, forexample (monocrystalline) silicon wafers, and indeed both in the case oflightly doped p-type and in the case of highly doped p-type wafers. Thisalso applies to boron, phosphorus, arsenic or other, including mixeddopings. Of course, the method according to the invention can also beadvantageously applied to other workpieces, for example monocrystalline,polycrystalline or glass-like workpieces made of silicon, siliconcarbide, aluminum oxide, silicate or other materials.

The workpieces are flat, preferably planar workpieces, which can becircular or even square. Wafers within a diameter range between approx.100 mm and 450 mm, with diameters staggered in accordance with the SEMIstandard of 100 mm, 125 mm, 150 mm, 200 mm, 300 mm, and 450 mm,including in particular silicon wafers having the diameter 300 mm, areparticularly preferred. Typical thickness ranges are in the range of 300μm to 950 μm, in the case of silicon wafers having a diameter of 300 mmin particular between 770 μm to 800 μm. The target thickness of theworkpieces striven for following the complete wafer processing processchain is typically between 770 μm and 780 μm and the workpiece thicknessduring double-side polishing is usually between 0 μm and up to 20 μmabove that due to the required allowance. The absorption of the opticalradiation in the workpiece varies, as does the optical density of thematerial, depending on, for example, the doping of the siliconworkpieces. Both, but in particular the latter, influence themeasurement result. If the corresponding workpiece property is known inadvance, the control apparatus can adjust the measurement methodaccordingly or modify the measurement result in a suitable manner, inorder to take account of these workpiece properties. To this end, it ispossible to divide workpieces into different classes and to allow anoperator of the double-side processing machine to select the class inthe control apparatus. Alternatively, the workpiece properties, forexample the doping concentration, could be stored as a numerical valuein the control apparatus which then adjusts the measurement result in asuitable manner. Another alternative would be measuring those workpieceproperties or properties correlating with them, for example conductivityin the case of the doping concentration of wafers, in the double-sideprocessing machine so that the control apparatus can, in turn, modifythe measurement result in a suitable manner. In particular, the opticaldensity of the material varies depending on the temperature of theworkpieces. This, in turn, influences the measurement result. If theworkpiece temperature is known during the measurement, this can, inturn, be taken account of in an appropriate manner in the controlapparatus. The storing of the temperature expected for the respectiveprocessing operation in the control apparatus, the classification ofdifferent temperature ranges and/or the selection of a temperature rangeby the operator would be conceivable. It is also possible to usetemperature measurement values of temperature measuring apparatuseswhich go to the control apparatus. In this case, for example,temperatures of the cooling media which are regularly supplied to theworking disks are possible. These can be measured, for example, by meansof suitable measuring apparatuses in the inlet or outlet of thedouble-side processing machine or also in the coolant reservoir.Temperatures, which are measured by means of suitable measuringapparatuses in, on or near the working gap, can also be taken as abasis.

It is understood that the optical workpiece thickness can in particularbe determined according to the invention. If the refractive index isknown or ascertained, the (mechanical) workpiece thickness can then bedetermined from this.

According to the invention, the measured workpiece thickness can, onreaching or falling below a certain workpiece thickness, above all beused to terminate the current processing operation or to transfer to asubsequent processing operation. A processing program specifiedaccording to the control apparatus can, to this end, include multipleprocessing operations which can be successively terminated in each caseon reaching or falling below a previously specified target thickness ofthe workpieces, or transferred to the next processing operation. Suchprocessing operations can follow each other immediately or withinterruptions. Of course, the measured workpiece thickness can also beused to determine the current workpiece removal rate and adjust processparameters during the workpiece processing in a suitable manner, forexample by means of an algorithm, in order to bring the removal rate toa predefined level.

The plurality of data, which the at least one thickness measurementapparatus transmits to the control apparatus, are preferably evaluatedwith statistical or general mathematical methods, data filtering,averaging, extrapolation or trend determination or other dataevaluation, in particular with the aid of algorithms, such that theevaluation produces good time-resolved representatives of the workpiecethickness. Such algorithms include an alteration of the measurementresults preferably on the basis of a calibration and corrections asdescribed above.

According to a particularly preferred configuration, the at least onethickness measurement apparatus can measure the thickness of theworkpieces by means of an interferometric thickness measurement method.Using interferometric measurement methods, the workpiece thickness canbe determined in a particularly precise manner. The prerequisite for theoptical radiation of the at least one thickness measurement apparatus ispartially transparent and partially reflecting workpieces whichtherefore allow a relevant portion of the optical radiation to passthrough the workpiece and back. An interferometric thickness measurementmethod is, for example, known from the aforementioned WO 2010/037452 A1.This method can generally be used within the context of the presentinvention. In this case, optical radiation is directed at the top sideof the workpiece, wherein a first radiation portion is reflected on thetop side and a second radiation portion penetrates the workpiecethickness, is reflected on the bottom side of the workpiece and emergesagain on the top side of the workpiece. The first and the secondradiation portions then interfere under formation of an interferencepattern. Using this interference pattern, the optical workpiecethickness between the top side of the workpiece and the bottom side ofthe workpiece can be determined in the manner described in WO2010/037452 A1. If the refractive index is known or established, forexample as described in WO 2010/037452 A1, the mechanical workpiecethickness can, in addition, be determined. It is also possible to directan infrared radiation spectrum at the top side of the workpiece, whereinthe radiation created by interference of the radiation portions can beanalyzed by means of a spectrometer.

An optical radiation source of the at least one thickness measurementapparatus can in particular emit infrared radiation. This isparticularly favorable during the thickness measurement of, inparticular, highly doped silicon workpieces, that means siliconworkpieces specifically provided with foreign atoms such as boron orphosphorus. The optical radiation source preferably emits infraredradiation in the wavelength range between 1050 nm and 1600 nm, becausesilicon is particularly transparent in this wavelength range. Thiswavelength range is more preferably between 1150 nm and 1350 nm.

At least one focusing optic of the at least one thickness measurementapparatus can be arranged in the at least one through-hole. A reductionof the distance between the focusing optic, which can for examplecomprise suitable lenses or similar, and the workpieces to be measuredincreases the light output and therefore the attainable signal quality.In particular, workpieces having a high degree of light absorption canonly be determined in the case of sufficient light output withsufficient precision. The thickness of the working disk can already beproblematic in this context. It is therefore advantageously envisaged inthe case of this configuration that the focusing optic is introducedinto the working disk and is therefore a short distance from theworkpiece.

According to a further configuration, the focusing optic can have afocus depth of at least 1 mm, preferably at least 2 mm. One difficultyof the thickness measurement method is the changing distance between thefocusing optic and the workpiece. In particular, the thickness of theworking coating which is usually located therebetween, such as apolishing cloth, of the working disk, varies depending on the type ofworking coating and the wear. However, the workpiece thickness which isdetermined by measuring technology is to be determined with a deviationif at all possible of less than +/−0.1 μm, in particular approx. +/−0.05μm independently of these circumstances. Therefore, in the case of thisconfiguration, a focusing optic is envisaged which has a large focusdepth. In order to increase the attainable accuracy, the focusing opticcan furthermore be mechanically adjustable such that the focus depthfully encloses the process area, that means the area in which theworkpiece is located during the different processing conditions, orplaces said process area into the center of the focus depth. Themechanical adjustment can, in this case, be implemented such that it canbe carried out, for example, during interruptions between processingoperations. In principle, an actuated mechanism which is used during theprocessing operation or in accordance with an adjustment instruction,which includes additional parameters such as, for example, a polishingcloth thickness, is also feasible.

The at least one through-hole can be flushed with compressed air in thearea of its entry to the working gap. It is also possible to generate anexcess pressure with respect to the working gap in the at least onethrough-hole. Both serve to protect the thickness measurement apparatus,in particular a focusing optic, from contamination from the working gap.This protection can also be combined with a shutter which only releasesthe light path for the duration of a measuring operation.

However, it is particularly preferred if at least one protective windowwhich is at least partially transparent to optical radiation from anoptical radiation source of the at least one thickness measurementapparatus is arranged in the at least one through-hole between thethickness measurement apparatus and the working gap. The protectivewindow can, in particular, be substantially completely transparent tothe radiation from an optical radiation source. The thickness of such aprotective window is preferably within a range between 0.5 mm and 20 mm,particularly preferably between 2 and 10 mm. In particular, in the caseof silicon workpieces, materials that are transparent in the range ofinfrared radiation are suitable for the protective window. Preferredmaterials for the protective window are aluminum oxide or calciumfluoride. A high chemical resistance of the protective windows inparticular to alkaline media, which are typically used as polishingagents in double-side polishing machines, is also advantageous.Generally, advantageous properties are a low temperature expansion or atemperature expansion similar to the working disk material, a highscratch resistance and a low tendency to fractures.

According to another configuration, the at least one protective windowcan be set further back with respect to the surface delimiting theworking gap of the working disk provided with the at least onethrough-hole by not more than 10 mm, preferably not more than 3 mm, morepreferably not more than 1 mm, most preferably not more than 0.3 mm. Thefact that the surface of the protective window on the workpiece side islocated close to the working disk surface means that the space betweenthe working gap and the protective window is quickly filledhomogeneously with a working medium, for example a polishing agent,during operation. In addition, the working medium flows evenly, lessturbulently and forming few bubbles past the protective window.Otherwise, this could lead to unstable measurement results. The factthat the protective window is only set back slightly can also be usefulfor helping to avoid interfering measuring signals, e.g. due to aworking medium film, e.g. a polishing agent film, between the workpieceand the protective window. The thickness thereof would then be limitedaccordingly and a measurement signal can be distinguished and filteredout from the signal for the workpiece thickness.

In another embodiment, the at least one protective window can be setback with respect to the surface delimiting the working gap of theworking disk provided with the at least one through-hole by at least 2mm and at most 10 mm. Other than in the case of the configurationexplained above, the protective window could also be advantageously setback to a greater extent, at least 2 mm in the configuration describedabove, so that the working medium film would be particularly thick. Thethickness thereof would then be limited at the bottom and anymeasurement signal which might occur could then be distinguished andfiltered out from the signal for the workpiece thickness.

It should also be pointed out with respect to the aforementionedconfigurations that the protective window does not have to have a flatsurface, in particular in the direction of the working gap. Thenumerical values indicated above then relate to that part of theprotective window which protrudes furthest being set back. In thisrespect, it may suffice if a majority of the light is guided through thearea of the protective window, which area corresponds in terms of itsgeometry to the embodiments described above.

The at least one protective window can be cleaned from the side of theworking gap by a cleaning apparatus using a cleaning fluid. In additionto or instead of flushing the working gap with a free-flushing processmedium, for example deionized water or an alkaline solution, during orshortly before the end of the processing operation, the protectivewindow can also be freely flushed following a processing operation,between two processing operations or before a processing operation witha cleaning apparatus. This can be done manually under low pressure.However, a method in which the protective window is regularly freelyflushed from the workpiece side with a pressurized cleaning fluid, e.g.with deionized water or an alkaline solution, is preferred. This can bedone automatically by the control apparatus. This can preferably be thesame apparatus which is used to freely flush or refresh the workingcoatings of the working disks. The space between the working gap and theprotective window can also be deliberately supplied during theprocessing operation with a liquid, preferably with water, an alkalinesolution or the working medium itself, in order to address the problemdescribed above that the working medium flows past the protective windowin a turbulent manner and forming bubbles.

According to another configuration, it can be envisaged that theworkpiece thickness is optically measured during the processing of theworkpieces by means of multiple through-holes which are configured inthe upper working disk and/or the lower working disk. It is in principlepossible to carry out the thickness measurement through the differentthrough-holes (for each working disk) with a joint thickness measurementapparatus, wherein the radiation from an optical radiation source of thethickness measurement apparatus can then be divided up in a suitablemanner, for example by means of beam splitters. However, it is of coursealso feasible that multiple thickness measurement apparatuses arearranged, to this end, on the upper working disk or on the lower workingdisk. The workpiece thickness can be measured through the differentthrough-holes simultaneously during processing. However, it is alsopossible to measure the workpiece thickness through the differentthrough-holes at different times during processing. i.e. staggered fromeach other.

As explained above, multiple workpieces can be simultaneously processedin the double-side processing machine. In the process, measuring errorscan be sorted so that measured values, which were ascertained while aworkpiece was actually in the light path of the optical thicknessmeasurement apparatus, are only or preferably taken into account. It isthen additionally possible to clearly associate the measurement resultsof the at least one thickness measurement apparatus in each case to oneworkpiece. This requires knowledge of the position and, at best, alsothe twisting of the rotor disks and the working disks in high temporalprecision. However, this information is available depending on themachine configuration. In addition, the number of measured values canalso be restricted in other ways, for example by pulsed thicknessmeasurements, wherein the radiation pulse can be provided by means of ashutter in the light path, an electrical circuit or sorting on the basisof an algorithm.

According to another configuration, it can be envisaged that the atleast one thickness measurement apparatus is arranged in avibration-dampened manner on the upper working disk and/or the lowerworking disk. Due to the mounting in a vibration-dampened or, at best,vibration-insulated manner of the thickness measurement apparatus, forexample a spectrometer of an evaluation unit of the thicknessmeasurement apparatus, a falsification of the measurement results can bereduced or avoided by vibrations. Vibrations with typical frequenciesbetween 10 Hz and 1000 Hz, in particular between 100 Hz and 1000 Hz,occur in the processing operation concerned here. The vibrationdampening can be an active, electronic vibration dampening or a passivevibration dampening, for example an elastic mounting, adjusted to therespective frequency band. In this way, a mechanical or thermallyinduced mechanical bracing between the working disk and the thicknessmeasurement apparatus, for example of an evaluation unit of thethickness measurement apparatus, can be avoided.

The control apparatus can be arranged in a separate location from theworking disks. The data transfer between the at least one thicknessmeasurement apparatus and the control apparatus and the electricalsupply of the at least one thickness measurement apparatus can then takeplace via at least one sliding contact. An evaluation unit of the atleast one thickness measurement apparatus arranged on the upper and/orlower working disk regularly has an optical grid and, for example, a CCDsensor. On the one hand, the data transfer, i.e. the transfer ofelectrical measuring signals, between the thickness measurementapparatus located on the rotating working disk and the non-rotatingcontrol apparatus of the double-side processing machine, which isarranged separately from the working disk, must be ensured. A slip ring,in particular having gold contacts, can be used as a suitable slidingcontact. On the other hand, the same is also true of the electric supplyof the thickness measurement apparatus. The signals can thereby beexchanged directly or indirectly or via a BUS system, for exampleProfiBUS or ProfiNET.

One exemplary embodiment of the invention will be explained in greaterdetail below with reference to the figures, wherein:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a schematic view of an embodiment of a double-sideprocessing machine,

FIG. 2 illustrates a histogram showing workpiece thickness deviationusing a method according to the prior art, and

FIG. 3 illustrates a histogram showing workpiece thickness deviationusing the current method.

Unless otherwise indicated, the same reference numerals denote the sameobjects in the figures.

DETAILED DESCRIPTION OF THE INVENTION

The double-side processing machine shown in FIG. 1 can, for example, bea double-side polishing machine. The double-side processing machine hasan upper working disk 10 and a lower working disk 12 opposite the upperworking disk 10. Part of the upper working disk 10 and the lower workingdisk 12 can, in each case, be a working coating 14, for example apolishing cloth. With this working coating 14, the upper and lowerworking disks 10, 12 delimit between them a working gap 16 formaterial-removing processing of workpieces. The reference numeral 18shows, by way of example for illustrative purposes, a workpiece, forexample a wafer. It is understood that, for example, rotatably arrangedrotor disks can in practice be provided in the working gap 16, whichrotor disks each receive multiple workpieces for simultaneous processingin the working gap 16.

In the example shown a through-hole 20, in particular a through-bore, isconfigured in the upper working disk 10. A focusing optic 22 of athickness measurement apparatus that is attached to the upper workingdisk 10 is located in the through-hole 20. The focusing optic 22 can,for example, comprise suitable lenses. In the example shown, aprotective window 24 is arranged in the through-hole 20 between thefocusing optic 22 and the working gap 16. The other end of thethrough-hole 20 is illustrated by a suitable sealing element 26. Thethickness measurement apparatus additionally comprises a measurement andevaluation unit 28 which is attached by means of vibration dampers 30 tothe top side of the upper working disk 10. An optical radiation source,for example a laser, preferably an infrared laser, is arranged, on theone hand, in the measurement and evaluation unit 28. The radiationemitted from the optical radiation source is conducted via a fiber-opticcable 32, in particular a glass fiber, to the focusing optic 22 and isfocused by the latter through the protective window 24, which is atleast partially transparent to the optical radiation, onto the workpiece18. In addition, an optical radiation sensor, for example a CCD sensorand for example an optical grid, is additionally located in themeasuring and evaluation unit 28. The measurement and evaluation unit 28controls the radiation source in order to emit optical radiation. Thisis partially reflected by the top side of the workpiece 18, partiallyenters the workpiece 18, is reflected by the bottom side of theworkpiece 18 and then exits again at the top side after passing throughthe workpiece 18 again. The optical radiation coming back from the topside of the workpiece and the bottom side of the workpiece interfereswith one another and is conducted via the focusing optic 22 and thefiber-optic cable 32 to the sensor arranged in the measurement andevaluation unit 28. The measuring signals received by the sensor can beevaluated in the measurement and evaluation unit 28, in order todetermine the workpiece 18 thickness. In particular, the workpiecethickness can be ascertained interferometrically as described, forexample, in WO 2010/037452 A1.

The measured values arrive at the measuring and evaluation unit 28 via asignal line 34 at a rotary joint 36, for example a slip ring, which isprovided in the area of the drive shaft 38 of the double-side processingmachine, by means of which the upper working disk 10 is rotatinglydriven during operation. The rotary joint 36 is connected at its otherend via an additional signal line 40 to a control apparatus 42 (PLC) ofthe double-side processing machine. A power supply for the thicknessmeasurement apparatus is also shown with reference numeral 44. Thesupply is provided via a first power line 46, a further rotary joint 48,for example a slip ring again, which is in turn arranged on the driveshaft 38, and a second power line 50 connected to the thicknessmeasurement apparatus.

The workpiece thickness can be ascertained during processing in areliable and precise manner with the method according to the inventionwhich is carried out with this double-side processing machine. If thecontrol apparatus 42 establishes that a previously specified targetthickness has been reached on the basis of the measurement resultsprovided, the control apparatus 42 terminates the current processingoperation. The control apparatus 42 can subsequently start the specifiedsubsequent processing step or terminate the processing of the workpiecesaltogether.

For illustrative reasons, FIG. 1 only shows a thickness measurementapparatus which measures the workpiece thickness via only onethrough-hole. It is of course possible for the workpiece thickness to bemeasured through multiple through-holes simultaneously or at staggeredtimes, whether this is by means of multiple thickness measurementapparatuses arranged on the upper working disk 10 and/or the lowerworking disk 12 or only by means of a thickness measurement apparatus,the light path of which is divided up in a suitable manner, as describedin principle above.

FIG. 2 shows a frequency distribution of the thickness measured valuesascertained for the workpieces following processing or their deviationfrom the target thickness for a conventional double-side processingmethod without the thickness measurement according to the invention fora plurality of processed workpieces, in this case wafers. FIG. 3 shows afrequency distribution of the thickness measured values ascertainedfollowing processing or their deviation from the target thickness forthe same double-side processing machine, but in this case with thethickness determination according to the invention, for a plurality ofprocessed workpieces, in this case wafers again. It is obvious that thethickness deviation when using the process according to the invention isconsiderably lower than it is for the conventional method. Inparticular, double the standard deviation 2σ in the method according tothe invention is only 0.25 μm while it is 1.11 μm in the conventionalmethod.

1. A method for measuring a thickness of workpieces processed in adouble-side processing machine, the method comprising: processing theworkpieces in a working gap formed between an upper working disk and alower working disk, the upper and lower working disks configured torotate relative to one another and remove material from the workpieces;optically measuring the thickness of the workpieces during processing byat least one optical thickness measurement apparatus disposed on atleast one of the upper working disk or the lower working disk, whereinthe at least one optical thickness measurement apparatus is configuredto measure the thickness of the workpieces disposed in the working gapthrough at least one through-hole in the at least one of the upperworking disk or the lower working disk; supplying the measurementresults of the at least one optical thickness measurement apparatus to acontrol apparatus of the double-side processing machine; and terminatingthe processing of the workpieces once a previously specified targetthickness of the workpieces is reached.
 2. The method according to claim1, wherein the at least one optical thickness measurement apparatusmeasures the thickness of the workpieces by an interferometric thicknessmeasurement method.
 3. The method according to claim 1, wherein the atleast one optical thickness measurement apparatus further comprises anoptical radiation source that emits infrared radiation.
 4. The methodaccording to claim 1, wherein the at least one optical thicknessmeasurement apparatus further comprises at least one focusing opticdisposed in the at least one through-hole.
 5. The method according toclaim 4, wherein the focusing optic has a focus depth of at least 1 mm.6. The method according to claim 4, wherein the focusing optic has afocus depth of at least 2 mm.
 7. The method according to claim 1,wherein the at least one through-hole is flushed with compressed air atits entry to the working gap.
 8. The method according to claim 1,wherein an excess pressure with respect to the working gap is generatedin the at least one through-hole.
 9. The method according to claim 3,further comprising at least one protective window that is configured tobe at least partially transparent to optical radiation from the opticalradiation source of the at least one optical thickness measurementapparatus and is disposed in the at least one through-hole between theat least one optical thickness measurement apparatus and the workinggap.
 10. The method according to claim 9, wherein the at least oneprotective window is comprised of aluminum oxide or calcium fluoride.11. The method according to claim 9, wherein the at least one protectivewindow is recessed not more than 10 mm with respect to a surfacedefining the working gap.
 12. The method according to claim 11, whereinthe at least one protective window is cleaned from the surface definingthe working gap by a cleaning apparatus using a cleaning fluid.
 13. Themethod according to claim 1, wherein multiple through-holes are disposedin at least one of the upper working disk or the lower working disk andare configured to allow the thickness of the workpiece to be opticallymeasured during the processing of the workpiece.
 14. The methodaccording to claim 13, wherein the workpiece thickness is measuredthrough the multiple through-holes simultaneously during the processingof the workpiece.
 15. The method according to claim 13, wherein theworkpiece thickness is measured through the multiple through-holes atdifferent times during the processing of the workpiece.
 16. The methodaccording to claim 1, wherein the double-sided processing machine isconfigured to process multiple workpieces simultaneously, and whereinthe measurement results of the at least one optical thicknessmeasurement apparatus are allocated to one workpiece.
 17. The methodaccording to claim 1, wherein the at least one optical thicknessmeasurement apparatus is coupled to a dampener.
 18. The method accordingto claim 1, wherein the control apparatus is positioned away from theupper working disk and the lower working disk.
 19. The method accordingto claim 18, further comprising at least one sliding contact configuredto effect data transfer between the at least one optical thicknessmeasurement apparatus, the control apparatus, and an electrical supplyof the at least one optical thickness measurement apparatus.